In a collaborative air-ground robotic system, the large-scale terrain mapping using aerial images is important for the ground robot to plan a globally optimal path. However, it is a challenging task in a novel and dynamic field without historical human supervision. To alleviate the reliance on human intervention, this article presents a novel framework that integrates active learning and generative adversarial networks (GANs) to effectively exploit small human-labeled data for terrain mapping. In order to model the diverse terrain patterns, this article designs two novel diversity-sensitive GAN models which can capture fine-grained terrain classes among aerial image patches. The proposed approaches are tested in two real-world scenarios using our collaborative air-ground robotic platform. The empirical results show that our methods can outperform their counterparts in the predictive accuracy of terrain classification, visual quality of terrain mapping, and average length of the planned ground path. In practice, the proposed terrain mapping framework is especially valuable when the budget in time or labor cost is very limited.